Reading my wet lab coverage, you may have noticed something - I wasn’t in any of the pictures. That’s because I volunteered as a support acoustician to further the training I began when I interned for the Northwest Fisheries Science Center last year. In that role, I primarily sat off to the side in the acoustics lab, analyzing acoustic data from five years ago to complete my Hollings project, observed fishing operations from the bridge, and helped in the wet lab when I could. But out at sea now, my mentor Steve de Blois is teaching me how to fish based on active acoustics from start to finish - and I’m learning a lot.
There are two main types of acoustics, passive and active, but this research survey relies on active acoustics for fishing operations. On the bottom of the boat, five echosounders emit pings of sound, and these pings reflect off of objects that have different densities than the water surrounding them (e.g. fish, whales, krill, or even air bubbles!). These reflections, or echoes, are known as backscatter. Based on the distance between the echosounder and the backscatter, an image called an echogram is created, representing where these objects are in relation to the ship. We use these echograms to decide where to fish. If you want an even better breakdown of this process, check out NOAA Teacher-at-Sea Nick Lee’s blog on the same subject, where he covered acoustics on the walleye pollock survey this summer.
As I’ve found out, things look a little different when you’re helping with the day-to-day acoustics operations. The day of an acoustician is long, since the echosounders run continuously and at any moment we could see backscatter of the hake we’re looking for. Steve, who is lead acoustician this leg, gets up before sunrise every morning to begin scouting for hake sign. Right now, during the summer, that means being down in the acoustics lab somewhere between 5 and 6 in the morning. When he finds a potential spot, he then checks with Chief Scientist Alicia Billings and FEAT team lead Julia Clemons to see if it’s the type of hake sign they’re looking for.
On this particular cruise, there’s an emphasis on finding hake aggregations that are comprised primarily of age 1-hake or a wide diversity of ages. This makes it easier to test net selectivity and see if the amount of young fish we’re catching is representative of how many young fish were at depth. Luckily, an experienced acoustician can make a general prediction based on the sign as to what age of hake is forming the aggregation - though these aren’t axiomatic. Adult hake tend to stay deeper, closer to the bottom, and form carpets or snakes. Age-1s might form snakes as well, but they’re usually shallower or over open water. They can also form blips (the science team likes to call these types of signs cherries or tomatoes). Age-0, or young-of-the-year, can be recognized from spiky, narrow aggregations. All of this knowledge acousticians build off of extensive experience looking at acoustic data on other hake surveys and while back on land.
When we’ve isolated a sign that looks promising, the next step is to call the bridge to give an aim point. Steve has let me pick a few of these, and it’s kind of fun isolating where we want to fish on a given sign. The bridge officers use the coordinates, bottom depth, and target depth given from this aim point to decide on the best route to set up for fishing. They then announce to the entire vessel that we’ll be fishing soon. Depending on how long it’s been since the ship first passed over the aim point, it can take anywhere from 20 minutes to a couple hours to get the net in the water to fish. How promptly we fish depends on factors like weather conditions, remaining clear of other vessel traffic, and even making sure the crew and scientists can get a meal before fish come on board.
When the net does go in the water, a lot of collaboration is necessary on the bridge to make sure the rest of the trawl operation goes smoothly. We work with several officers and the chief boatswain to ensure the boat moves at the proper speed and alignment (and that the net goes down and opens wide enough) to hit the target and catch hake. This can result in a lot of anticipation - we all sit and wait and stare at the monitors as the boat creeps closer to the aim point.
When the deck officer feels that we’re close enough to the aim point to start, our focus narrows to just three screens - the net sounder, a display showing the amount of wire between the trawl doors and the ship, and an echogram showing real-time acoustic returns.
The echogram shows what’s happening directly underneath the boat, so that’s where we all look first. When a hake sign creeps on screen, that’s good news - and we turn our attention to the net sounder. This screen is split into two - the left side gives a vertical profile, with the top of the profile corresponding to the headrope, or the top of the net. A thick red line appears another 20-40 m down, corresponding to the footrope/bottom of the net. Anything that looks particularly fuzzy red between those two usually denotes biological sign - fish or zooplankton. The right side of the screen gives a better idea of the cross-section of the net, and updates every few seconds. If anything appears within the faint ring of the net, that means it’s going in. Usually fish appear as horizontal lines - Lieutenant Jaime Hendrix refers to these lines as “sprinkles.”
Sometimes the fish come in fast, sometimes more like a trickle. This is why the acousticians onboard cross-reference with the echogram - it gives them a better idea of what’s coming, how dense an approaching fish aggregation might be, and how they might need to adjust the net to meet an aggregation (or avoid one). If they have to adjust the net (bring it lower or higher in the water column), they ask the bridge officers to figure out how to drop or raise the net, and then the bridge officers will ask the chief boatswain to either wire out or wire in a certain number of meters. On this cruise, the goal is to take small samples of dense schools, so we don’t fish for very long. In fact, active fishing lasted for less than three minutes on two of the trawls I’ve led! There’s definitely a balance, but our hopes as scientists are to only take what we need to understand what type of hake are present and accomplish the objectives of the cruise. When the lead acoustician of the trawl is satisfied, we haul back the net and see what we get!
Learning how to fish has been a bit nerve wracking - choosing where and how long to fish based on 2D images and a flurry of sprinkles can be challenging. But I’ve had the great fortune of working with experienced and knowledgeable scientists, crew, and NOAA Corps officers. Working alongside them (and learning alongside them, since a few officers are also new to the process) has made each trawl exciting, and I look forward to going fishing again sometime soon.
About the author: Maddie Reifsteck is a volunteer with the Fisheries Resource Analysis and Monitoring Division of the Northwest Fisheries Science Center. She is a former Hollings Scholar and a recent graduate of Eckerd College, pursuing a career in marine science.
